Facebook cover photo size: 820 by 312

The numbers reduce badly. The greatest common divisor of 820 and 312 is 4, leaving 205:78, a ratio with no memorable form at all. Its decimal value is 2.6282, so the frame is wider than 21:9 and narrower than 3:1, which is exactly the sort of shape that does not interchange with anything else you already have.

At 0.26 megapixels it is by a long way the lowest resolution preset in this set, lower even than a 512 pixel favicon canvas in total pixel count. On a high density display a file at exactly these dimensions will look soft, so the practical approach is to work at the same ratio and a larger size, then let the platform downscale.

The crop differs between contexts. A desktop layout shows a wider, shorter slice than a phone does, and the two take different parts of the same file, so a composition that fills the desktop crop can have its subject cut off on mobile. Centring the essential content and treating the outer thirds as expendable is the only reliable defence.

There is no preset at this shape in the tools here. The cropper takes 820 and 312 as a custom width and height, and the calculator will confirm the ratio and the megapixel count before you commit the export. Given how little vertical room the strip has, checking the numbers before you start designing is far cheaper than discovering the shape at the end.